Cannot create an association <association name> - property listed twice
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Cannot create an association <association name>. The same property is listed more than once: <property name>.
Associations are defined by the selected Association Properties in the Association Editor dialog box. Properties can be listed only one time for each class in the association.
The property in the message appears more than one time in either the Parent or Child class's Association Properties.
To resolve this condition
Examine the message and note the property specified in the message.
Click OK to dismiss the message box.
Inspect the Association Properties and remove the duplicate entries.